I have HP PSC 2110 USB printer and I want to get cups and hpoj services to be started whenever I plug the printer (or put power on). Here is how I achieved this. This is easily modified to any usb device. Pre-requirements are that you have USB correctly setup and emerged hotplug, and usb drivers compiled as modules.

Kernel loads automatically correct usb drivers whenever you plug the device, you should see this from your system log (/var/log/messages or /var/log/everything/current). For me when I plug the printer it loads "printer" driver (module).